If a vehicle exhibits high NOx emissions along with LTFT at +18% and STFT fluctuating between +5% and +10%, what might be the issue?

In this scenario, the presence of high NOx emissions combined with a long-term fuel trim (LTFT) of +18% and short-term fuel trim (STFT) fluctuations between +5% and +10% suggests that the vehicle may be running lean. A faulty positive crankcase ventilation (PCV) valve can contribute to a lean air-fuel mixture by allowing excess air to enter the intake system, which can lead to an increase in NOx emissions.

NOx emissions are typically elevated when combustion temperatures are high, often associated with a lean mixture. The readings of positive fuel trims further indicate that the engine control module (ECM) is trying to compensate for lack of fuel, which is often a sign of an issue in the fuel delivery system or air intake system. In this case, a malfunctioning PCV valve could cause an unmetered air leak, affecting the air-fuel mixture and inadvertently driving up NOx emissions.

Other potential issues, such as a faulty catalytic converter or defective oxygen sensor, while they could affect emissions as well, are less directly associated with the specific symptoms provided regarding fuel trim values and the lean condition. A leaking exhaust manifold would also typically result in other signs, such as increased engine noise or exhaust